America’s weak economy has produced the broadest ideological spectrum in American presidential election history. As a result, both political parties have contests running completely opposite from the usual pattern. And because each party is enabling the other to avoid the consequences of running away from the nation’s ideological midpoint, this dual deviation is likely to continue right up to November.

In presidential campaigns, America’s two parties aim to run to the nation’s ideological center as soon as possible. The reason is simple: The center determines the winner. America’s ideological breakdown tells the story.

In 2012, national presidential exit polling showed liberals comprised 25 percent, conservatives 35 percent, and moderates 41 percent of the electorate. Neither Democrats with their liberal core, nor Republicans with their conservative one, can win without a considerable portion of moderates.

Each party establishment’s ideal is to start with a candidate from their side of the ideological spectrum, but one as close as possible to the center. The candidate with this promise attracts the party regulars, the major donations and the mainstream media attention. And if the party establishment has their way, he or she will grab the nomination and an inside track to the middle ground in the general election.

Deviation from this middle way spells disaster. Each party remembers its deviation-delivered landslide defeat: Republicans with Barry Goldwater in 1964 and Democrats with George McGovern in 1972.

So strong is the pull to the center — and so high the cost of straying from it — that each party’s inevitable and ultimate charge in the general election is that their opponent tilts too far from center. For Republicans, the charge against the Democrat is being liberal; for Democrats, the charge against the Republican is being “out of the mainstream.”

All this has usually held sway — until now. In 2016, each party has a strong challenge from its respective ideological pole. The result is a presidential campaign unlike a normal American election and more akin to a parliamentary one from abroad.

Certainly, history has produced candidates from across America’s ideological spectrum, but never ones combining such ideological extremes with such substantial support. For the Democrats, Sen. Bernie Sanders describes himself as a socialist. For the Republicans, Sen. Ted Cruz willingly embraces a far-right label. In past campaigns, such descriptions would have been leveled from a candidate’s opposing party. Now both are mounting serious challenges within their own.

And if Mr. Sanders and Mr. Cruz are from different ideological poles, Donald Trump brings a different dimension altogether. Mr. Trump takes the contest beyond just the left-right spectrum into a personal populism defying ideological description — and herein lies his appeal to a steadily growing number of dedicated supporters.

This election is unlikely to revert to the norm once nominations are settled. While past elections have been fought by each party seeking to contest the center, this one is going to be waged by each party seeking to drag the center toward its ideological pole. This is not the conscious decision of either party’s establishment, but the de facto result of the primaries’ expanded ideological spectrum.

Both eventual general election candidates will have had to take positions far closer than normal to an ideological extreme in order to win their party’s nomination. Even if neither nominee represents that ideological extreme, to retain their hard-won base and appeal to their vanquished party challengers’ supporters, they will have to retain more extreme positions in the general election. And each party is going to be enabled in doing so by the same dynamic taking place in the other party.

How long this expanded ideological spectrum will last in America’s presidential elections remains in question. However, it is likely to stay expanded as long as the issue driving it remains: America’s underperforming economy.

While many issues roil today’s electorate, none is as pervasive or powerful as the prolonged poor economy.

Following World War II, the American economic miracle reached full flower. From 1947 through 2000, real gross domestic product (GDP) growth averaged 3.4 percent. However over the past 16 years, it has averaged just over half that at 1.8 percent. This 16-year period spans a two-term presidency from each party. Americans’ dissatisfaction with both parties — and politics in general — is therefore no surprise.

Under the pressure of relative economic duress, American politics has spilled across the ideological spectrum. In doing so, it has undercut normal party politics and turned the prevailing logic of presidential campaigns on its head.

With its economy in a prolonged stagnation, America is experiencing a double deviation in its election-year norm. Both parties are being pulled further and harder toward their respective ideological pole than ever before. Past logic would argue that the first party able to reorient itself toward the ideological center will win. But thus far, 2016 has held little regard for past logic. Accordingly, this year the center may not hold up any better than the economy has.

• J.T. Young served in the Treasury Department and the Office of Management and Budget, and as a congressional staff member.